Best S/PDIF digital out to my NFB-11 DAC

highly subjective ranking based on the performance on three other audio-gd dacs with sabre chip (nfb-10es, nfb-1, nfb-7) -- should be applicable to nfb-11:

audiophilleo2 -- 10/10
hiface evo    -- 8/10

squeezebox touch -- 6/10
squeezebox receiver -- 4/10
spdif from nuforce's udac2   -- 3/10

http://www.amazon.com/Musical-Fidelity-MFVLINK-V-Link/dp/B004PH03GU

or 

http://www.audio-gd.com/Pro/dac/USBface/Digital1EN.htm

either of the above, will likely easily beat any sound card spdif. The v-link is right at your budget, the cheapest version of the audio-gd interface is about 170$ shipped.

I just upgraded from a X-FI Coax out (Flexi Jack 1/8 to coax) using a custom cable from bluejeans (1/8 to RCA) with their 1694a. 

to 

V-link and bluejeans 1694a spdif, with a belkin gold usb cable. 

It easily beats my x-fi, no problem.

EMU makes studio grade cards with spdif and optical out, i have the 1212m pci. It's a bit fiddly though to setup. $100 used.

MAudio makes nice stuff too.

Anytime.

I don't know that many cards have onboard transformer isolated outputs, but i think you can make your own pretty easily, if you have a soldering iron. I assume they would just be two RCA female terminations on a box with the transformer inside. What transformer to us I have no idea, have a look on google.

Well, if your budget is 100$ - the v-link fits perfectly at 99$ shipped with no tax. :)

im pretty sure the 1212m and even 0404 have transfomer coupled spdif output.

The Fiio E10 is also an usb to coaxial converter

esi juli@ pci soundcard

really high stable asio drivers and excellent spdif digital out up to 192 khz.

The quality of the spdif out is equal to lynx two as you can read here :

http://www.thomann.de/it/esi_juliat.htm

at the bottom of the page in spdif loop through comparisons

I have a V-Link between my Mac and my Burson.  I also have a HiFace card, which goes up to 192K, but it's not got the right output voltage (by design.  Bad idea on an otherwise solid device, as some systems can't take the input voltage it provides).  

For 96KHz output they sound the same to me.  If you need 192K, the HiFace is the cheapest USB-S/PDIF I know that can put it out with a good clock, but it does require a driver (after one glitch it's been fine on my Mac).

Ditto on the Emu cards.  I had the 0404 and 1212 pci cards and their spdif out is really good and they are cheap used.  Bit of  a pain to setup though.  The V- link is okay but you will need to spend a hundred or more on cables if you really want to hear what it can do.  And after that, it is still nowhere near as good as the Halide Bridge (that is way over your budget).

What about a M2Tech Hi-Face or variants, the TeraLink X2 is a good interface also.

Unless you are looking for the card to add extra features such as EQ...etc.

I suggest you try ESI Juli@ as one of the best in its price range for SPDIF output. Its direct competitor is Infrasonic Quartet that also gets very good reviews. Finally Audiotrak Prodigy HD should fit your price bracket. The disadvantage of the latter is that it only has toslink digital output and no coax, which I prefer more, but I do not know your preferences here.